It's common for the square inches of intake bore be larger with the R/C throttle than with the factory venturi, tho sometimes the factory venturi is still too large. What are you planning to do with the engine? Stunt, sport, SL Combat?  Steve

Santa brought an OS venturi (.260) and spraybar (.137) for my .25 FP (area of .156344).
I don't know how to calculate the area of the carb but the opening is .220, spigot .100 D x .100 L  (best I can measure).
I ran it in the bench today with the 40D carb that was on it when I got it and with the new goodies.
10x3 APC and 9x4 APC both ran ~1000 rpm less with the venturi setup.
Is that as it should be?

   I haven't tried that, but it wouldn't overly surprise me. For stunt, the venturi is about as big as you want it to be. BTW, the spigot makes it possible to draw fuel acceptably well with more choke area or, if you like, better with the same choke area. That's why you want one for your regular engine.

     The only issue with running the RC carb is that it occasionally just changes speed, probably due to the barrel shifting or small, varying, air leaks.

     Brett

My FP clone magnums ive got 1/4 bore ( ,250 ) with a 4 mm ( 5/32 in. ) spraybar . about right on 9 x 6s in the twin .

You pull 10.000 ?

you got a 10 x 4 ? ish

With the tank inside the spraybar , in plan , shell richen level , lean off high . Max Lean top of wingover .

Trial Set nose straight up , full lean , then out a click or 3 to suit . or more .

Brett - what do you mean by "regular engine"?
To clarify, I'm thinking the carb area *may* be smaller than the venturi/spraybar area, yet it allows a good bit more rpm.
The carb may actually be larger though, since I can't calculate it.

    Regular engine = full-tilt stunt engine like a PA or Jett. The spigot venturi was FAR AND AWAY the biggest improvement we ever found on the PA61, in particular. Several of us were ready to give up until David "discovered" it - that is, "discovered" why Billy's engine ran so much better than his at the WC in Germany. 

         A straight .220 bore has much more choke area than a .257 with a .135 spraybar across it, so it makes sense that even with the spigot, it would have more choke area, and give you more RPM in a bench test. I have several of the RC carbs, I can take a shot at calculating the choke area.

    Brett

OK, thanks for clearing that up, Brett, and I'd appreciate the calculation.
I don't know if the engine ran better with the carb because it had more or less area than the venturi.
I can lock the carb barrel and seal it to minimize irregular running.

   I would just try it. If it goes overly lean in the maneuvers, too fast overall, or you have to back the engine off too much to get the right in-flight speed, then it's too big. Much of what is "commonly understood" about stunt engines is well-meant, but nonsense. Ultimately, in the large, you pick the venturi that provides the right in-flight speed at the correct "setting".

    I might actually test-fly the airplane with the carb. I haven't done that in a long time and never with the 25LA.

     Brett

    I haven't tried this with an LA.25, but I have an LA.40 on my Dusty scale/stunt model. For scale competition, I have a small box that is covered to match the color scheme and it hold a small servo, battery and receiver for the throttle. I can fly the pattern with the throttle set but it uses too much fuel that way, and it doesn't run consistently. I change over to a venturi and an OS. needle valve for pure stunt (.250 or .257, I can't remember for sure.) On the venturi, the engine runs completely different, more like you might expect for a stunt engine and runs very steady and makes good power and hauls the 48 or 50 ounce air frame with ease. I use a Master Airscrew 10-5 three blade for a prop in both modes. Because of the experience I have with this engine, I think the carb opening is much larger than the venturi I chose, which I based on what I read on this forum that most people were using, and it doesn't have any restriction like the venturi does. It still needs a full, 4 ounce tank to do the pattern, but the engine is still quite new and might not be fully run in yet. The carb barrel on my engine is pretty sloppy. Seems to run in R/C mode OK and idles well, but I think it is this poor fit that causes the different running. I would expect the LA.25 to act similarly. Good luck with the project!
   HAPPY NEW YEAR!
   Dan McEntee

Yea , just try the spigot one .

Try a few props etc . On the 35/40 at least , the rc 7 cl mufflers are differant , so theres room there too .

My FSR Magnum ran a 7.5 dis with 1/8 spraybar & leaky NVA on a 10 x 6 clipped to 9 1/2 . 9 x 6 or 4 in hurricanes .

This is a size intake usually used on .46s , 15.2 mm . So you never know till you try it .

If you trying to get by with an RC carb to save the expense of a venturi & NVA you might try this.

Lock the carb at something less than 100% open.  That should give you the fuel draw of a smaller venturi. 

Actually, guys, at this point I'm just curious about the carb producing more rpm than the venturi/NVA - whether the carb area is greater and so runs better or the venturi area is too large so doesn't run as well or what?

As long as you're getting a good mixture, mo air = mo power.  The fact that the carb gives more power indicates that air is shooshting through it easier than the venturi.

According to this article, and this one, too, the 25LA mostly suffers from breathing problems.  You can get LOTS more power out of it by opening things up.  BUT -- that's absolutely positively not what you want for stunt.  For us, it's just a point of mild interest (unless you're in the Northwest and want to come play with us in Geezer Speed).

If it makes a difference I have an FP, Tim, and my interest at the moment is more academic, but on another thread some have suggested the .25 FP may not be enough engine for a Banshee so it may become a more practical issue when I finish the plane.

Sorry, forgot to mention -- I think that you can treat the 25FP and the 25LA as identical for the purposes.

The 25FP may or may not be enough for a Banshee -- I don't know.  I think that was the thread where I suggested that you make sure you can swap in a 46LA (or a Tower 40), but try the 25FP.

However, you DO NOT want an engine designed to put out it's maximum power.  What you want is an engine that has enough power reserve to pull the thing up the vertical lines, while still being "willing" to reduce its power output in level flight.

'I spent a month in Aussie plus spent quite a lot of time with Aussies here, so I'm at a loss.'

But Steve, Matt's a Kiwi. Maybe that explains it.

That Theres 15.2 Sq mm AREA , free . In the intake at the min/ spraybar .

The olde Pi x R squared trip .http://www.mathgoodies.com/lessons/vol2/circle_area.html
 Remembering R is HALF the D ( diameter ) so a verneer or drill bit measures the bore of the intake , we use HALF that at cucalate the AREA of the orrifice ,
by subtracting from that the area of bits bloking it , so a 4 mm dia spraybar in a 6.5 mm hole is 4 x 6.5 ( approx cos its round at the ends ! ) so - 26, call it 25 .

Crikey , , so half of 6.5 = 3.25 as ' R ' , x 3.14 = X itself = The Answer , minus the 25 bit .

With the big intake I think it only worked 4-2 as the Needle DID leak air in with the fuel . As when I sealed it with a bit of fuel tube on the needle/ spraybar there , it just ran in an even two .

THUS if I hadnt tried it with the loose rattly needle , I wouldve missed out big time . As It's had 10 + gallons thru it like that , and only a few tanks ' sealed .
( the sping/ clickers adjusted lightly against the needle, it holds the setting fine . Rolling a finger will adjust it , tho picks up finger on , drops to ' new ' setting finger off .
only need 2 or 3 clicks variation , as a rule . Bending the Vent ( the ends cut at 45 degrree ) so straight forward into the airstream , I had to put it 1/4 turn IN , for same setting .
That only lasted one sesion , worked better ( engine a lot steadier ) with the vent tangental to the airstream  ) .

THUS , Id drag both your intakes ( Carb & Intake ) To The Field , And Try BOTH . As youll never know what your missing , unless you Try It . !  


Re Bretts post BELOW , often on R.C. carbs you / I  can LOCK the Barrel by tightening the Idle Stop Screw right in , till it bottoms . GENTLY . the Just Firm . One of those screws anyway .  

Pic of the Nasty FOX BB 25/19 Intake , Line Bobbins used as seals Ea Side .

15.2 mm = .5984". Are you meaning the venturi bore or the bore of the intake boss in the crankcase, or is it a typo? What is a "7.5 dis"? I seldom understand Matt's posts. I spent a month in Aussie plus spent quite a lot of time with Aussies here, so I'm at a loss.  Coffee Steve

OOPS ,

Dis is a Typo .should read DIA > ,
 15.2 sq mm is from memory
 7.5 squared off 'd be about 6 mm SQUARE .   thus 6 x 6 'd be 36 , minus the spraybar , 6 x 3.2 in this case . 18.6 ! so calcs a bit rough , but indicative .leaves 17.5 ish
True Calc is 15.2 . Square mm AREA , Not DIAMETER .

They robbed us of ' imperial ' measurements ', in ' New Maths ' back in 68 . Metrics , dollars & cents instead of pounds shillings & pence , in Newzealand . AND Stopped the free Milk at Schools !

I measured down the throat of the carb, Brett, so must not have gotten the caliper tips into the barrel bore itself.
I'll recheck with a small hole gauge (shoulda done that to begin with  ) but your figure is probably correct.
As Matt noted, I lock the barrel with the stop screw, tho' I rotate the barrel 180 so the screw bears on the surface of the barrel, not in the cut. This prevents axial or rotary movement. I also run the airbleed screw in fully, on the assumption it closes completely.

I rechecked the barrel bore and it is .220, but this is an OS 40D carb.
The bore of the 2A carb on my .20 FP measures ~.180.
So if my math is correct, a .220 bore w/a .1 x .1 spigot gives an area of ~.280 sq in - a good bit larger than the venturi/NVA.
With the interchangeability, I have a wide range available to try on the Banshee.

   0.0280, but yes, that's pretty darn big. I anticipate that it will not work very well, likely going dead lean after launch, or early in the flight. But only one way to find out.

    Brett

Wow, Phil, talk about braindead - I never thought of that possibility. 

Wow, Phil, talk about braindead - I never thought of that possibility. 

   Of course, this will potentially solve it. Adjusting the venturi area instead of the needle is a generally better way to adjust the speed, and with both adjustments available, you can play one off against the other very easily (compared to having to swap the venturis around). Of course that also introduces an additional way to either fix it, or mess it up...

    Brett

Terry,

(Disclaimer, I only fly for fun, not competition)
Regarding the suggestions to lock the R/C carb in an intermediate position, here's a photo of what I do. This carb is on an OS .45 FSR and does not have a locking feature. I set the mixture with the engine running at full opening on the carb (slightly rich). Then I pull the carb back to get about 10,000 rpm. I visually note the position of the carb barrel and linkage rod position and kill the engine by putting my finger over the carb opening. Then mark the rod position with a Sharpie pen (I mark the linkage rod aligned with a bolt position on the engine mounting lugs). Fabricate the linkage and you're good to go. Adjust the aircraft speed with different props until you find one that works for you. This engine starts easily, but occasionally I have to flip it with my finger partially chocking the carb.

Paul